Video displays can be classified as either reflective or transmissive. Both types of displays are quite sensitive to lighting conditions. Liquid crystal displays used in portable computers and other electronic devices, such as paging devices, telephones, telefacsimile machines, watches, instruments and the like, are examples of low-power, reflective video displays which can be seen clearly in bright lighting conditions and depend upon reflection of ambient light, off a high contrast display surface, for their appearance. Reflective displays become difficult or impossible to view clearly in dim or low lighting conditions, for example, at night.
Transmissive displays include a light-emitting source, for example phosphors or diodes, which source can be built into the display screen. Common examples are conventional television screens and computer monitors as well as, for example, portable computer displays that employ light-emitting diodes. Another important application for light-emitting, transmissive displays is in instrumentation for aircraft, automobiles and other vehicles. Light-emitting transmissive displays can be clearly seen in low lighting conditions but lose contrast and visibility in bright light, for example, when taken outdoors. Safety problems may occur when operators view vehicle instruments or other travel guidance displays, during dramatic changes in ambient light. Thus there is a need, for example, for aircraft flight information displays which remain clearly visible as an aircraft emerges from heavy cloud into the brilliant sunlight of the stratosphere. Another desirable lighting-independent display is a computer screen, especially a portable computer screen that can be viewed outdoors.
Over a period spanning several decades, I have developed electronically driven video displays that employ, as pixels, light-modulating capacitors with movable electrodes. In a light modulating capacitor, a movable electrode is coiled, or otherwise prestressed, into a compacted, retracted position from which it can be advanced across a dielectric member by application of a drive voltage. The drive voltage is controlled by a fixed electrode on the other side of the dielectric member, the movable and fixed electrodes and the dielectric member constituting a variable capacitor.
By appropriate choice of the optical character of the movable electrode, a light beam striking the capacitor can be modulated as desired, for example by varying the hue, lightness and saturation characteristics of the light. Using tapered electrode patterns or shapes, the extent of excursion of the electrode can be made voltage dependent and thus controlled. Arrays of such pixels can be matrixed to provide large-screen displays such as highway signs and scoreboards. As disclosed in my above-cited pending applications, electrostatically driven pixels can be miniaturized and incorporated not only in conventional video-displaying equipment such as computer and television equipment but also in novel, compact low-power flat-screen displays that can, for example be small enough to be handheld and carried in a pocket.
My U.S. Pat. No. 3,989,357 shows some early examples of such light-modulating capacitors, employing a rolling electrode which is coiled in its retracted position and which is deployed in a variety of embodiments, including matrixed assemblies of multiple such elements. Several reflective display embodiments are shown, which rely upon ambient light to illuminate the display. In addition, the embodiment of FIG. 5 shows a light-transmissive pixel, in which light passes through a somewhat translucent fixed electrode of such a capacitive element. This FIG. 5 proposal depended upon a thin layer of aluminum to be both translucent and adequately conductive to act as a fixed control electrode for a movable light-modulating electrode. Since aluminum is only mildly transparent to light when thick enough to be conductive, the use of an aluminum film electrode has limited application and is not satisfactory for pictorial quality video displays.
My patent No. 4,266,339 relates to a method of manufacturing rolling-electrode-type, light-modulating capacitors, which method employs heating and cooling of a metalized tented plastic sheet to provide prestressed, coiled electrode elements or spirals.
My patent No. 5,231,559 discloses novel video displays; and their manufacture, including video displays employing light-modulating capacitors with transparent electrodes utilizing a light path to a viewer which traverses at least one transparent electrode on both an incident and a reflected course. High transmissivity electrode material, for example indium tin oxide, is used to avoid undue attenuation of the light beam as a result of its dual passage through the transparent fixed electrode. These capacitors can be assembled into more or less complex pixels, some employing two or more such movable light-modulating electrodes along a single light path. Multiple electrodes can visit the same area across a light path and can be assembled in pairs, or other groupings, on either side of a transparent substrate. Employing such matrixes of flexibly configurable pixels, full color and monochrome reflective screens can be created which have excellent definition, excellent visibility, are economical and easy to manufacture from low cost materials, yet consume very little power.
Some further developments of these subjects are disclosed and claimed in my pending application Ser. No. 08/066,949 including digitally responsive display pixels, manufacturing methods, miniaturized electrode configurations and some non-video applications of the inventive electrostatically actuated movable electrode technology. Further patents of mine describe and claim reflective display devices which employ electrostatically driven, active movable elements that have become known as "flapper" devices because they move from one side to another of a reflective channel to switch between different display states having selected, different visual characteristics. Some such patents are U.S. Pat. Nos. 4,488,784, 4,094,590, 4,336,536 and 4,468,663. These disclosures are not applicable to transmissive display pixels.